As the incidence of obesity rises dramatically in the United States, more and more severely obese individuals are undergoing surgical treatment to reduce body weight and associated risk factors. Roux-en-Y gastric bypass (RYGB) is now the most common operation to treat morbid obesity in the US. However, little is known about the effects of RYGB on body composition and resting energy expenditure (REE). The main objectives of this study are to determine: 1) the composition of weight loss following surgery, 2) whether protein supplementation and strength training can limit the expected reduction of lean mass and REE. The study candidates will be morbidly obese women with a body mass index (BMI) of 40-56 kg/m2, be 18 - 49 y.o. and premenopausal. Except for severe obesity, they will be relatively healthy with a history of diet failure. They will be sedentary except for walking. There will be 36 study participants who, after stratifying by race, will be randomly assigned to three treatment groups (n = 12): 1) standard postoperative nutritional counseling only, 2) protein supplementation and standard postoperative nutritional counseling, or 3) protein supplementation plus strength training and standard postoperative nutritional counseling. The protein supplementation will begin shortly after surgery and increase from 40 g/day to 80 g/d at 4 weeks. Strength training will begin 8 weeks postoperation, to allow for adequate wound healing, and will consist of twice weekly progressive resistance training for upper and lower body for a period of 12 weeks. A battery of test measurements following a 12 h overnight fast will be conducted prior to surgery and repeated postoperatively at 8 and 20 weeks. These tests will include measurement of REE and body composition using underwater weighing, air displacement (BODPOD), dual xray absorptiometry (DEXA), magnetic resonance imaging (MRI), isotope dilution (D20), sodium bromide, and regional anthropometrics. There also will be assessments of arm and leg strength. Additionally, there will be measures of fasting glucose and body weight related hormones insulin, leptin, cortisol, and the recently discovered ghrelin. The predictions are that during the dramatic weight loss after surgery, the loss of some lean tissue, which could adversely impact skeletal muscle and vital organs, will be reduced by enhanced protein intake and weight training. There also may be greater conservation of REE and bone density. Plasma glucose and hormones should all decrease, especially with exercise, except for ghrelin, which should increase. The findings should improve understanding of surgical weight loss in morbidly obese patients and have clinical applications in the postoperative care of such patients.